Studies continue on the role of the virus in chronically infected murine Friend virus-induced leukemic cell lines (FLC) which differentiate along the erythroid pathway under certain conditions. When FLC are cultured in the presence of dimethyl sulfoxide (DMSO) (2 percent) or dimethyl formamide (DMF) (1 percent), differentiation and hemoglobin synthesis are stimulated, and there is a decrease in malignant potential. The number of viruses budding from the cell surfaces, however, is increased. The viruses harvested from the control as well as DMSO-supplemented tissue culture supernatant fluid have little or no leukemogenic activity. Experiments are in progress to characterize the biochemical properties of the two virus strains to determine whether any differences can be detected to account for the loss of leukemogenicity.